Strictly heterogeneous reaction

Ion-exchange processes are strictly heterogeneous reactions that involve two phases. They, however, mostly comply with homogeneous, second-order reaction kinetics. An exchange reaction represented by... 

In systems with concentrations that are held constant, the speeds with constant space function - which is the term ( ) of product isothermal conditions. This is true for the volumetric speed of a homogeneous system as well as for the areal speed of a catalytic system. This will be also true in the case of a strictly heterogeneous reaction that obeys the law of < E. Space functions are, however, frequently a function of time, which leads to absolute speeds or to rates that vaiy with time through this space function. 

A novel application of a symmetric porous membrane as a catalyst carrier but not as a permselective barrier is to use the membrane itself as the reaction zone for precise control of the stoichiometric ratio [Sloot et al., 1990]. In this case, the reactants are fed to the different sides of the membrane which is impregnated with a catalyst for a heterogeneous reaction. The products diffuse out of the membrane to its both sides. If the reaction rate is faster than the diffusion rate of the reactant in the membrane, a small reaction zone or theoretically a reaction plane will exist in the membrane. An interesting and important consequence of this type of membrane reactor is that within the reaction zone the molar fluxes of the reactants arc always in stoichiometric ratio and the presence of one reactant in the opposing side of the membrane is avoided. The reaction zone can be maintained inside the membrane as long as the membrane is symmeuic and not ultrathin. Therefore, membrane reactors of this fashion are particularly suited for those processes which require strict stoichiometric feed rates of premixed reactants. A symmetric porous a-alumina membrane of 4.5 mm thick was successfully tested to demonstrate the concept [Sloot et al., 1990]. 

The opposing reactant contactor mode applies to both equilibrium and irreversible reactions, if the reaction is sufficiently fast compared to transport resistance (diffusion rate of reactants in the membrane). This concept has been demonstrated experimentally for reactions requiring strict stoichiometric feeds, such as the Claus reaction, or for kinetically fast, strongly exothermic heterogeneous reactions, such as partial oxidations. Triphasic (gas/liquid/solid) reactions, which are limited by the diffusion of the volatile reactant (e.g., olefin hydrogenation), can also be improved by using this concept. 

Rate constants of heterogeneous reactions are usually represented in the three-parameter form discussed above. However, this form has a clear physical sense only for reactions in ideal gases at a strictly kept equilibrium Maxwell-Boltzman distribution. Despite several attempts to adopt transition-state theory to heterogeneous reactions, and to those proceeding in adsorbed layers in particular (e.g., Krylov et al., 1972 Zhdanov et al., 1988), its applicability in these cases is doubtful. 

It would be very desirable to use the a constants, which have been carefully determined for homogeneous reactions in solution, for the reactions of solid catalysts also. This would overcome the difficult task of finding suitable catalytic transformation as a standard for the definition of special catalytic a values such a process must exhibit high reproducibility which is difficult to attain with heterogeneous catalytic reactions. However, the application of the liquid-phase a values requires the same blend of interaction mechanisms to operate in both homogeneous and heterogeneous reactions. This a strict condition which might not be always met. 

Remember that the rates are not strictly equal. Therefore it would be wrong to write their difference, in other words the overall rate, as equal to zero. To give a numerical example, if the forward rate were equal to 1 000001 and the backward rate equal to 1 000 000 (in mol m s ), the overall rate, equal to 1, is different from zero. The same type of confusion is made by authors who use the term local equilibrium to refer to a reversible heterogeneous reaction . 

In certain biosensors an irreversible heterogeneous reaction is transformed into a homogeneous redox reaction via a very reversible redox system. The latter can be transformed back at the working electrode by a much lower overpotential, thereby reducing the chance of co-oxidation or co-reduction of interfering compounds. Since the limiting current is strictly proportional to the analyte concentration and also to the concentrations of interfering compounds, chemometric data treatment and/or differential measurements may be used to correct for errors. 

Strictly speaking, reactions in the bulk of aerosol particles are homogeneous reactions (e.g. liquid phase reactions), but here the general convention of calling non-gas reactions "heterogeneous is followed. 

This discussion is certainly an over-simplification. Unfortunately there are no detailed experimental results for this reaction under strictly homogeneous conditions, but even with heterogeneous catalysts (e.g., AlCl3 and Ni [13]) only mixtures of branched paraffins, naphthenes and polyenes of low molecular weight are obtained. If isomerisation is slower than propagation, as indicated, e.g., by the experiments of Meier [5] on the polymerisation of 3,3-dimethyl butene-1, this would modify in detail but would not invalidate the above general conclusions. 

It should be noted that solid explosives may be detonated in any condition from a coarse powder to a single crystal (Ref 6, p 166). Heterogeneous polycrystalline mixtures can be termed "solid only by convention phenomena such as grain erosion in the detonation reaction zone are of dominant importance. They depend in a complex way on the intercrystalline free space and on a free space more strictly defined, the difference between the volume of the crystals and the volume of the ions therein... 

All of the above discussion is strictly applicable only to homogeneous gas phase reactions. Usually the above considerations do apply reasonably well to non-polar liquids and nonpolar solutions, although normal Z values may be an order of magnitude less than for gas reactions. Reactions in solids are often much more complex, since they are usually heterogeneous, involve catalytic effects, reactions at preferential sites (dislocations, etc), and nucleation phenomena. These complicated processes are quite beyond the scope of the present article. For some description of these phenomena, and further references, the reader should consult Refs 9, 10 11... 

In general, carbides, nitrides, and borides are manufactured in the vapor phase in order to form high-purity powders. This procedure is fundamentally different than a strict CVD process, since in powder synthesis reactors, deposition on seed particles may be desirable, but deposition on the reactor walls represents a loss of product material. As we will see, in CVD, heterogeneous deposition on a surface will be sought. Aside from this issue of deposition, many of the thermodynamic and kinetic considerations regarding gas phase reactions are similar. 

Strictly speaking, mechanisms for heterogeneous catalytic reactions can never be monomolecular. Thus they always include adsorption steps in which the initial substances are a minimum of two in number, i.e. gas and catalyst. But if one considers conversions of only surface compounds (at a constant gas-phase composition), a catalytic reaction mechanism can also be treated as monomolecular. It is these mechanisms that Temkin designates as linear (see Chap. 2). 

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